Exact Date Pinned to Great Pyramid's Construction?

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The Egyptians started building the Great Pyramid of Giza on August 23, 2470 B.C., according to controversial new research that attempts to place an exact date on the start of the ancient construction project.

A team of Egyptian researchers arrived at the date based on calculations of historical appearances of the star Sothis—today called Sirius.

Every year around the time of the Nile River floods, Sothis would rise in the early morning sky after a long absence.

"The appearance of this star indicates the beginning of an inundation period" for the Nile, said team leader Abdel-Halim Nur El-Din, former head of Egypt's Supreme Council of Antiquities.

Throughout their history, "Egyptians started their main buildings, the tombs, and the temples at the beginning of the inundation"—an auspicious time, since floodwaters brought fresh soil, maintaining the region's fertility.

In addition, pharaohs always started building their tombs at the starts of their rules. Khufu, the pharaoh meant to be buried in the Great Pyramid, took power in 2470 B.C., according to Nur El-Din and colleagues.

The researchers therefore compared the modern calendar, the ancient Egyptian calendar, and the cycle of the star to find the exact day Sothis would have appeared that year.

The team believes the ancient Egyptians observed the star from July 17 to 19, and the inundation period began 35 days later—on August 23.

Using Sothis's arrival to keep track of the annual Nile floods made sense, said Mark Hammergren, an astronomer at the Adler Planetarium in Chicago who was not involved in the work.

"It happens at about the same time every solar year, so it corresponds to the seasons, and it provides them a good fixed date," he said.

Hammergren agrees with the days Nur El-Din's team calculated for Sothis, based on other researchers' estimates for the dates the star would have risen during ancient Egyptian times.

Still, Hammergren noted, the "appearance" of the star is subject to weather conditions, which might have obscured its first rise in any given year.

Mahmoud Afifi, the general director of Giza antiquities, is also concerned about placing an exact year on the start of Khufu's rule.

Ancient Egyptian chronology reset to zero at the beginning of each pharaoh's reign, making it difficult to match Western calendar years with the dates given for ancient events.

What's more, dated lists of kings are unreliable, since the ancient Egyptians often had political motivations to alter the historical record.

Some unpopular pharaohs could have been left off the lists, for example, which would have changed the ruling dates of every pharaoh that followed.

Many scholars debate the precise year Khufu ascended to the throne, with some estimates as much as 139 years earlier than the date Nur El-Din and his team selected.

In addition, the design of the monumental Great Pyramid probably took considerable time to prepare, Afifi said, which might have delayed the start of construction beyond the first year of Khufu's reign.